SEM analysis of PVDF-HFP nanofibers for the fabrication of energy harvesters

Nowadays, energy harvesting is seeing an increasing interest from the research community, a fact that is confirmed by the rising number of publications. Energy harvesting has a wide range of applications, ranging from portable electronics, such as wristbands, to implanted medical devices like pacemakers. In this field, researchers are focusing their attention on the development of new energy harvesters that satisfy strict requirements: they need to be light and small, but also cheap and highly portable. In this blog, we discuss the fabrication of energy harvesters made from PVDF-HFP nanofibers on PDMS and SF substrates. We investigate how these energy harvesters are characterized and what the role of SEM is in this study.

Piezoelectric Energy Harvesting

The increasing demand for innovative devices, such as embedded sensors in sportswear or smart watches, is drawing attention to energy harvesting. Energy harvesters have the capacity to convert external energy, which can be derived for instance from solar power or thermal energy, into electrical energy that can be used to power small electronic devices or wireless sensor nodes. Energy harvesters need to be small, light, inexpensive, portable, flexible and, in some cases, also biocompatible.

One of the most common types of energy harvesters employs piezoelectric materials, which convert mechanical strain (for example human motion or acoustic noise) into electric current or voltage. A commonly-used piezoelectric material for energy harvesting applications is polyvinylidene fluoride (PVDF), which offers a good electro-mechanical coupling factor and is biocompatible, light and flexible.

In a recent study, polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) nanofibers were investigated as suitable candidates for energy harvesters (R. Najjar et al., Polymers 2017, 9, 479). The performance of the nanofibers was characterized in combination with two different substrates, namely polydimethylsiloxane (PDMS) and silk fibroins (SF).

The first is a type of synthetic polymer, while the second is a natural protein that provides better biocompatibility and more favorable sustainability. The characterization of the performance of energy harvesters includes the analysis of the morphology, the mechanical properties, and the mechanical-electrical measurements.

Characterization of PDMS and silk substrates through SEM analysis

A scanning electron microscope was employed for the analysis of the morphology of PDMS and silk films. For this analysis, two types of silk fibroin films were investigated: pure silk fibroin and silk fibroin with 20% glycerol content.

Because silk fibroins tend to become stiff and brittle over time, glycerol is added to the silk fibroin to make it more flexible. 20% is the optimum glycerol content for increasing the softness of the silk film, without the film disassembling in water.

The study of the mechanical properties of the three types of film is also of utmost importance. Figure 2 shows the stress-strain curves of the three materials. The PDMS (blue curve) is mostly elastic with a linear stress-strain curve until the fracture, showing a maximum stretch of more than 400% its total length, whereas the pure silk fibroin (pink curve) is stiffer and has lower yield point than PDMS.

Fig 2: Stress-strain curve of PDMS and two types of silk fibroin films.

The data from the measurements on silk fibroin prove that this material can survive larger forces and greater elongation, although it is stiffer than PDMS.

SEM analysis of PVDF nanofibers

The PDVF-HFP nanofibers were fabricated using the electrospinning process. Two different types of fibers were produced: random nanofibers and aligned and stretched nanofibers. Figure 3 shows SEM images of the two types of nanofibers (A and B).

From these images the fibers diameter and orientation can be measured. In Figure 3, the diameter distribution for random and aligned fibers is shown (graphs C and D respectively). In the first case, the diameter varies between 600 nm to 1600 nm, while for aligned fibers it ranges from 300 nm to 700 nm.

The orientation distribution (shown in graphs E and F) shows that the random fibers have a larger range of orientation (from -50° to +50°), while the aligned fibers have orientation with one large peak around 0°.

Finally, the energy harvesting measurement was performed. Figure 4 shows the voltage generated from PVDF-HFP random (A) and aligned (B) nanofibers on a PDMS substrate. The voltage generated from the stretched and aligned nanofibers is more than 12 times that of the electrospun random nanofibers.

The electro-mechanical characterization was important in demonstrating that the aligned PVDF-HFP nanofibers have higher piezoresistivity and are therefore more suitable for energy harvesting applications. The SEM revealed to be a powerful instrument to analyze the morphology of the nanofibers and to measure the fiber diameter and orientation.

From that, stretched nanofibers were shown to be better aligned with a more precise diameter control. They also outperformed the random nanofibers in the energy harvesting measurement by more than 10 times.

If you are curious about nanofibers and their applications, we recommend you download this case study on polymeric nanofibers. It was written in collaboration with the Department of Nonwoven textiles, from the Technical University of Liberec (CZ).

The case study details how the use of the Phenom desktop SEM, combined with the FiberMetric software application, improved workflows and the research program in general.

About the author

Karl Kersten is head of the Application team at Thermo Fisher Scientific, the world leader in serving science. He is passionate about the Thermo Fisher Scientific product and likes converting customer requirements into product or feature specifications so customers can achieve their goals.